WASHINGTON - US airline system traffic in May fell slightly after rising for four months, according to the data released by US Transportation Department on Thursday.

US airlines carried 63.7 million domestic and international passengers in May, 0.3 percent fewer than the same period of last year, yet 3.6 percent more than that in May 2010, said the department.

The department said there were 0.4 percent fewer domestic passengers and 0.7 percent more international passengers in May than a year ago.

Southwest Airlines was ranked the first among American airways by the total number of 11.72 million passengers it hauled in May, of which, domestic passengers made up 11.62 million, while United Airlines, following its merger with Continental Airlines, was placed the first among the US airlines by transporting 2.16 million international passengers in the same month.